It is a type of drier which comprises a hollow rotary cylinder which at least in operation is angled so that its ends are at different heights relative to the ground. In general, the angle of the axis is approximately several degrees relative to the horizontal.
Connected to one end of the cylinder there are heating means normally consisting of a burner which generates a flame that extends inside the cylinder.
The combustion fumes then pass through the rest of the cylinder and reach a chimney, usually connected to the end of the cylinder opposite the end connected to the burner.
Also connected to the two ends of the cylinder there is an infeed section through which the aggregates to be dried are inserted, and an outfeed section through which the treated materials are extracted from the cylinder.
Depending whether the infeed section is connected to the end to which the burner is connected or to the other end, the drier is referred to as cocurrent (since the direction of feed of the fumes and of the material is the same) or counter-current (since the direction of feed of the materials is opposite to that of the fumes).
However, irrespective of the type of drier, the infeed section is always connected to the cylinder at the end which in operation is higher above the ground, so that the combined effect of cylinder rotation and angling causes the material to be fed through the cylinder.
Inside the cylinder there are usually many series of blades designed to mix and feed the material being processed as well as to facilitate heat exchange.
In particular, the blades intended only for feed can adopt a very spiral shape relative to the axis of rotation, whilst those also intended for mixing and/or heat exchange normally extend at least mainly parallel with the axis of rotation.
Depending on their structure, the blades for mixing and/or heat exchange may generally be divided into tipping blades and containment blades. The former are blades characterised in that they have a mouth for the material whose width is significantly greater than the depth of the blade (understood to be the distance between the edge of the blade and its innermost point) as well as a profile which prevents the formation of undercuts. Said blades are designed to collect the material as they pass in the lower rotation zone and to pour it out so that it showers through the combustion fumes passing through the central part of the cylinder.
With a suitable construction it is possible to unload more than 80% of the material contained in the tipping blades practically just after they have reached the highest point of the rotation (only at that moment is their mouth facing downwards). In contrast, the containment blades are blades in which the width of the mouth is generally comparable (the same as or slightly less than/greater than) the depth and they have a rounded profile forming an undercut able to retain the material. These blades are designed to minimise the quantity of material unloaded to shower through the combustion fumes. With the shape described above it is possible to ensure that during rotation they pass the highest point having unloaded even less than 20% of the material initially loaded.
The inside of the cylinder is axially divided, starting at the first end, into a first heat exchange zone, in which heat exchange occurs mainly by convection, and a second heat exchange zone, in which heat exchange occurs mainly by radiation and conduction. The different heat exchange is achieved by using tipping blades in the first heat exchange zone where the temperature of the fumes is lower, and containment blades in the second heat exchange zone where the temperature is significantly higher due to the presence of the flame.
As regards recycled materials, plants for the production of bituminous macadams usually use materials obtained from cutting existing road surfaces, which are normally mixed with new aggregates in predetermined proportions.
For that reason, the driers for which the present invention is intended comprise an insertion section for inserting recycled material into the cylinder, the insertion section being connected to an intermediate portion of the cylinder. In particular, the insertion section may or may not be connected to the cylinder at a change in its diameter.
According to the prior art, the insertion section is positioned between the first and second heat exchange zones, so that the recycled materials are subjected to heating mainly by conduction and radiation.
Also according to the prior art, the insertion section comprises one or more radial openings made in the wall of the cylinder and a feeder for directing the recycled material to the openings from the outside. Inside the cylinder, there may be a tubular structure coaxial with the cylinder and designed to prevent the entering recycled material from passing directly through the combustion fumes, diverting it at a tangent along the lateral wall of the cylinder (see for example patent EP 1 624 109).
However, all types of prior art driers (whether of the counter-current or co-current type) have disadvantages.
In particular, all prior art plants have limits regarding the possibility of using recycled material. Above predetermined limits of approximately 15-20%, the bitumen contained in the recycled material usually causes the material to become packed together, attaching to the blades and the cylinder.
A second disadvantage of prior art plants is the fact that they cannot guarantee good mixing of the hot aggregates and the cold cut material which is added, meaning that the temperature distribution is very uneven in the cut material, causing the formation of emissions which are harmful to the environment.